Sympathetic postganglionic neurons typically release nor-epinephrine, which excites some effector cells but inhibits others. The receptors on target cells may be either α-or β-adrenergic receptors. These receptors are further subdivided into α1, α2, β1, β2, and β3 receptor types on the basis of pharmacological and genetic features. The distribution of these types of receptors and the actions that they mediate when activated by sympathetic postganglionic neurons are listed for various target organs in
Table 11.1 α1 receptors are located postsynaptically, but α2 receptors may be either presynaptic or postsynaptic. Receptors located presynaptically are generally called autoreceptors; they usually inhibit release of transmitter. The effects of agents that excite α1 or α2 receptors can be distinguished through the use of antagonists to block these receptors specifically. For example, prazosin is a selective α1-adrenergic antagonist, and yohimbine is a selective α2-adrenergic antagonist. The effects of α1 receptors are mediated by activation of the inositol triphosphate/diacylglycerol second messenger system (see
Chapter 3).
In contrast, α2 receptors decrease the rate of synthesis of cyclic adenosine monophosphate (cAMP) through action on a G protein.
β receptors were originally classified on the basis of the ability of antagonists to block them, but this has been supplemented by genetic studies. The β1 and β2 proteins have been much more extensively studied than has β3, but it is thought that the proteins that make up all three types of β receptors are similar, with seven membrane-spanning regions connected by intracellular and extracellular domains (see ). Agonist drugs that work on β receptors activate a G protein that stimulates adenylyl cyclase to increase the cAMP concentration. This action is terminated by the buildup of guanosine diphosphate.
Radialmuscle,irisα Contraction(mydriasis)++ —Sphinctermuscle,irisα —Contraction(miosis)+++ Ciliarymuscleβ Relaxationforfarvision+ Contractionfornearvision+++
Sinoatrialnodeβ1Increaseinheartrate++ Decreaseinheartrate;vagalarrest+++ Atriaβ1IncreaseincontractilityandconductionDecreaseincontractilityand(usually)velocity++ increaseinconductionvelocity++ Atrioventricular(AV)nodeβ1IncreaseinautomaticityandDecreaseinconductionvelocity;AVblockconductionvelocity++ +++ His-Purkinjesystemβ1IncreaseinautomaticityandLittleeffectconductionvelocity+++ Ventriclesβ1Increaseincontractility,conductionSlightdecreaseincontractilityvelocity,automaticity,andrateofidioventricularpacemakers+++
Coronaryα,β2Constriction+;dilation
Skeletalmuscleα,β2Constriction++;dilation c,e
Pulmonaryα,β2Constriction+;dilation
Abdominalviscera,renalα,β2Constriction+++;dilation + —Salivaryglandsα Constriction+++ Dilation++ Veins (Systemic) α,β2Constriction++;dilation++ —
Responses of Effector Organs to Autonomic Nerve Impulses—cont’d 11.1
Effector Organs Receptor Type Adrenergic Impulses,a Responsesb Cholinergic Impulses,a Responsesb
Bronchialmuscleβ2Relaxation+ Contraction++ Bronchialglands?Inhibition(?)Stimulation+++
Motilityandtoneα2,β2Decrease(usually) + Increase+++ Sphinctersα Contraction(usually)+ Relaxation(usually)+ SecretionInhibition(?)Stimulation+++